1. Field of the Invention
The present invention relates generally to signal receivers and more particularly to a signal receiver which has a mono-pole antenna.
2. Description of the Prior Art
FIG. 1 shows an example of a prior-art doublet antenna (or dipole antenna) type signal receiver.
The doublet antenna type signal receiver of FIG. 1 is provided with a cabinet 1 made of a plastic material and with rod antennas 2 and 3. The rod antennas 2 and 3 are connected through a balun 4 to a tuner (not shown) which is provided inside of the cabinet 1.
FIG. 2 is an equivalent circuit of the doublet antenna type signal receiver shown in FIG. 1. The antennas 2 and 3 are connected through the balun 4 to an input impedance R of a tuner. Within the cabinet 1, there exists disturbing noise sources such as a microcomputer, a switching source, various digital circuits or the like which generate electrical fields hereinafter referred to as an electric field disturbing signal sources and signal disturbing sources which generate magnetic fields (hereinafter referred to as magnetic disturbing signal sources). The disturbing signal sources are represented by e.sub.1 and e.sub.2 in FIG. 2.
FIG. 3 shows the distributions of the electric fields produced by the electric field disturbing signal source e.sub.1 and also shows the magnetic field produced by the magnetic field disturbing signal source e.sub.2. Although a small disturbing current flows through the tuner which is generated by the disturbing signal sources e.sub.1 and e.sub.2, since there is no disturbing signal source e.sub.3 (see FIGS. 4 and 5) which is caused by a voltage drop which is generated by a current which flows through an earth or ground pattern such as an inside circuit which will be described later, the disturbance is very small.
Although the above-mentioned doublet antenna type signal receiver does not pick up its own disturbance very easily, it needs two rod antennas 2 and 3 to operate and this is inconvenient and is a disadvantage for a compact-sized signal receiver.
FIG. 4 shows an example of a prior-art mono-pole antenna type signal receiver which has a mono-pole or single rod antenna which removes the defects encountered with the doublet antenna type signal receiver shown in FIG. 1.
FIG. 4 shows a cabinet 5 made of a plastic material, a rod antenna 6 and an earth or ground pattern 7 which connects an earth portion of, for example, a printed circuit board and one portion of a metal chassis to ground. As shown in FIG. 4, the earth pattern 7 is used as one antenna which is opposed to the rod antenna 6 which is the mono-pole antenna, and a terminal E of the earth pattern 7 and a terminal P of the rod antenna 6 are connected to a tuner which is included in a circuit portion 8.
FIG. 5A shows an equivalent circuit of FIG. 4 which includes another disturbing signal source e.sub.3 in addition to the disturbing signal sources e.sub.1 and e.sub.2 because the earth pattern 7 is used as one of the antenna.
FIG. 5B shows a substantial equivalent circuit of the electromotive forces e.sub.1 ', e.sub.2 ' and e.sub.3 ' which correspond to the respective disturbing signal sources e.sub.1, e.sub.2 and e.sub.3 and in which e.sub.1' represents an equivalent electromotive force formed by the electrostatic coupling between the earth pattern 7 and the magnetic field disturbing signal source e.sub.1. e.sub.2 ' represents an electromotive force of the magnetic field which is induced in the earth pattern 7 by the magnetic field disturbing signal source e.sub.2, and e.sub.3 ' represents a voltage drop caused by the current flowing through one portion between I and J (see FIG. 4) of the earth pattern 7. The voltage drop e.sub.3 ' is the disturbing electromotive force which is produced in series in the earth pattern 7.
FIG. 5C is a diagram of an equivalent circuit which illustrates the electromotive forces corresponding to the respective disturbing signal sources. In FIG. 5C, e is expressed as e=e.sub.1 '+e.sub.2 '+e.sub.3 '. FIG. 6 is a diagram of the distribution of electromotive forces (displacement currents) of the disturbing electric field which are generated by the disturbing signal source e.
In the case of the prior-art mono pole antenna type signal receiver shown in FIG. 4, described above, the disturbing signal sources e.sub.1, e.sub.2 and e.sub.3 are equivalently connected in series to the earth pattern 7 which forms one of the antenna and the electromotive forces of the electric field are distributed as shown in FIG. 6 so that one portion of the displacement currents from the disturbing signal source e flows to the input impedance R of the tuner through the antennas 7 and 6. In other words, the electromagnetic field caused by the higher harmonic components produced from the digital circuit or the like mounted inside of the signal receiver is picked up by its own antenna so that the signal receiver is subjected to a disturbance which is so-called autointoxication. As a result, a beat interference occurs in a television picture which causes line flicker in the television picture and a beat interference occurs in an audio signal which causes the sound quality to be deteriorated. Further, these beat interferences cause the synchronization relationship to be broken so that the signal receiver malfunctions